A blockage of a blood vessel is the most frequent cause of stroke and is responsible for about 75 percent of the nearly 150,000 U.S. stroke deaths each year. Stroke ranks as the third leading killer in the United States after heart disease and cancer. There are 500,000 to 600,000 new strokes in the United States each year. As many as 3 million Americans have survived a stroke with more than 2 million of them sustaining some permanent disability. The overall cost of stroke to the nation is $30 billion a year.
A carotid endarterectomy is a surgical procedure in which a doctor removes fatty deposits from one of the two main arteries in the neck supplying blood to the brain. Carotid artery problems become more common as people age. The disease process that causes the buildup of fat and other material on the artery walls is called atherosclerosis, commonly known as “hardening of the arteries.” The fatty deposits are called plaque, and the resulting narrowing of the artery is called stenosis. The degree of stenosis is usually expressed as a percentage of the normal diameter of the opening. Carotid endarterectomies are performed to prevent stroke. Two large clinical trials supported by the National Institute of Neurological Disorders and Stroke (NINDS) have identified specific individuals for whom the surgery is highly beneficial when performed by surgeons and in institutions that can match the standards set in those studies. The surgery has been found highly beneficial for persons who have already had a stroke or experienced the warning signs of a stroke and have a severe stenosis of 70 percent to 99 percent. In this group, surgery reduces the estimated 2-year risk of stroke by more than 80 percent, from greater than 1 in 4 to less than 1 in 10. In a second trial, the procedure has also been found highly beneficial for persons who are symptom-free but have a severe stenosis of 60 percent to 99 percent. In this group, the surgery reduces the estimated 5-year risk of stroke by more than one-half, from about 1 in 10 to less than 1 in 20.
A stroke occurs when brain cells die because of decreased blood flow to the brain. In some cases, small pieces of plaque in the carotid artery may break loose and block an artery in the brain. The narrowed opening in the carotid artery can be a source of blood clots that travel to the brain, can trap blood clots from other areas of the body, or can become completely clogged.
U.S. Pat. No. 4,867,141 discloses a medical treatment apparatus which utilizes ultrasonic energy for medical treatment, particularly to break up a stone formed in a living body. An endoscopic channel is used to insert a portion of the apparatus into a body cavity, where an ultrasonic transmission member is used to transmit ultrasonic vibrations to the stone which is in contact with the distal end of the apparatus. A perfusion liquid is supplied to the area of the stone as the stone is being broken up by mechanical ultrasonic vibrations. This perfusion liquid is suctioned away from the area of the stone. As a result of the suction, the perfusion liquid and broken pieces of the stone are drained away from the body cavity. The apparatus of that patent is configured with an ultrasonic transmission member which is aligned and coaxial with the central axis of the probe, and therefore is effective in treating conditions—such as stones—where the irregularity or condition to be removed is aligned with the body vessel through which the endoscope passes. The device of that patent is used on non-hydrated calcified tissue, and uses direct mechanical vibration of the calcified tissue in order to result in tissue fracture and destruction.
U.S. Pat. No. 5,176,677 discloses an endoscopic ultrasonic rotary electrocauterizing aspirator. The background section of that patent includes some discussion of medical literature relating to prostatectomies, and in particular the Krawitt et al. technique, in which a gland can be removed using ultrasonic treatment without effecting to the prostatic capsule. However, the apparatus shown in that patent is disclosed as being useful for arthroscopic surgery. The apparatus shown in that patent includes a feature for providing irrigating fluid to the tip of the ultrasonic probe, as well as mechanisms for aspirating the area around the tip. The aspiration and irrigation features of that invention require individual passageways coaxial with the ultrasonic working tip, each connected to a source of pressurized fluid, for irrigation, or to a source of reduced pressure, for aspiration. The apparatus of that invention also includes other features adjacent the tip, such as an insulated hood for removing obstructions, and a telescopic viewing apparatus. The irrigation, aspiration, insulated hood and telescopic viewing apparatuses all increase the cross-sectional profile of the apparatus. The design of that apparatus also is such that it may only treat areas which are directly axially in front of the ultrasonic probe, and therefore which are axially aligned with the lumen or incision through which the probe is inserted.
Various other patents show apparatuses which use ultrasonic energy to fragment or transform body tissue. U.S. Pat. Nos. 5,112,300; 5,180,363; 4,989,583; 4,931,047; and 3,805,787 each show ultrasonic treatment apparatuses for use in treating various medical conditions. In each of these patents, some mechanism is shown for providing irrigation and/or aspiration in the area where the ultrasonic treatment is being performed. In each of these patents, however, the mechanisms for irrigation or aspiration are structured such that they increase the overall cross-sectional profile of the instrument. In addition, in each of those patents, the irrigation and aspiration ports are a fixed distance from one another, which may not be varied.
“Ultrasonic processing,” as used in the prior art for, inter alia, orthopedic surgery, is a technique wherein a body—either liquid or solid—is, in effect, “blasted” by ultrasonic energy. In ultrasonic processing, the ultrasonic energy produced by the ultrasonic vibrator influences water molecules found within the body tissue. The ultrasonic energy is in the form of very intense sound vibrations at a very high frequency. These intense, high-frequency sound vibrations result in powerful chemical and physical reactions in the water molecules within the body tissue. The reactions in the water molecules ultimately results in a process called “cavitation,” which can be thought of as a form of cold (i.e., non-thermal) boiling of the water in the body tissue, wherein there is a rapid creation and collapse of numerous microscopic bubbles in the water.
The result of cavitation in water is a “breaking” of that fluid. The rapid vibrations in water caused by the application of ultrasonic energy to the water and the resultant cavitation can cause fatigue in the water molecules which will break bonds between the water molecules. The result is that the water changes from a liquid form into a gaseous form, i.e., converts into steam, but this conversion is done without the need for application of thermal energy to the water. The result is a “cold boiling” of the water.
When a steam bubble is created in a cold liquid, such as upon the application of ultrasonic energy to water, the steam will condense because it is surrounded by a cold liquid. As a result, a void or cavity is created. The surrounding water molecules rush in to fill that cavity; when they reach the center of the cavity, they collide with each other with great force. This process is called cavitation. Cavitation is a known phenomenon which results in shock waves running outward from the collapsed bubble. The shock waves caused by cavitation can wear away or destroy material. For example, such shock waves are known to wear away metal at the edges of an outboard motor propeller.
Ultrasonic processing or ultrasonics is the application of sound at extremely high intensity and high frequency (normally above human hearing; 20 kHz and above) so as to result in material changes. Ultrasonics are used in a number of different applications in order to change a variety of different materials. Ultrasonics accelerates both physical and chemical reactions in the materials to which ultrasonic energy is applied and these reactions, among many other things, are accomplished largely due to the action of cavitation. There are more actions inherent in bubble collapse which are of significance. As used herein, the term “bubble” refers to a space within a liquid which contains a gas or vapor. However, after that gas or vapor condenses, there is still a void or cavity in that space until an implosion occurs. Therefore the term “bubble,” as used herein, also refers to the void or cavity.
One description of the manner in which cavitation is used in medical applications has been provided by Professor Lawrence Crum of the Applied Physics Laboratory at the University of Washington in Seattle, and can be found at the website: <http://nero.apl.washington.edu/harlett2/artgwww/acoustic/medical/medical.html>. Professor Crum, writing about lithotriptry—in which a kidney stone is broken with ultrasonic energy—has stated that “[w]hen pressure surrounding a bubble falls below the vapor pressure of the liquid, the bubble fills with vapor and grows explosively. The bubble collapses violently when pressure returns. If the collapse occurs near a boundary, such as [a] targeted kidney stone, a high velocity liquid jet is formed that impacts the boundary with great force. These extremely violent processes are thought to play a major role in stone destruction and associated tissue damage.”
In addition to erosion or ablation of surfaces by the jet, cavitation causes many other actions. Notable among these in a purely physical sense is the action of intense shock fronts generated by imploding cavitation bubbles against kidney stones (lithotriptry), gall stones, tumors, and other intrusions in the body. Some of this action can also be accomplished by direct impact of a vibrating ultrasonic tool tip, but no (or minimal) cavitation is involved.